This invention relates to tunable optical filters.
In many optical applications, it is desirable to use a tunable optical filter, such as an etalon, to modulate the intensity of narrow-band light. A tunable optical filter, which is a bandpass filter, is typically made up of two partially reflective mirrors or surfaces separated by a gap to form a cavity. Devices with this structure are called etalons. The spectral characteristics of a tunable optical filter are generally determined by the reflectivity and gap spacing (cavity length) of the mirrors or surfaces. Varying the effective cavity length of the device tunes the center wavelength of the spectral bandpass of the etalon. The effective cavity length may be varied by altering the actual physical gap size, the refractive index of the gap medium, or both.
Various micro-electromechanical system (MEMS) based tunable optical filters have been investigated for the purposes of cost-effective miniaturization and batch fabrication, but the tradeoff between optical performance and MEMS miniaturization limits their commercialization.
U.S. Pat. No. 5,022,745, incorporated herein by this reference, describes an electrostatically deformable single-crystal mirror having a highly-conductive thick substrate layer supporting a highly-conductive thin membrane. The membrane is separated from the substrate by an insulating layer. The center of the insulating layer is etched away to form a cavity. The outer surface of the membrane is polished and coated with a dielectric material to form one mirror. A voltage is applied between the membrane and the substrate to cause the membrane and its mirror to deform.
This prior art design has numerous deficiencies. First, because the dielectric layer is directly attached to the deformable membrane, the potential exists for the mirror to curl. Second, the highly conducting membrane itself is disposed within the optical volume. The optical volume is the region through which light travels during operation of the device. The highly conducting membrane can absorb certain wavelengths which therefore cannot be selected for filtering. Also, the dielectric layer on the membrane will deform with the deformable membrane, causing errors. Moreover, the optical portion/path and the actuating structure are physically and electrically coupled and consequently there is a tradeoff between the substrate thickness and elastic stiffness (factors which partially determine the actuation voltage).
U.S. Pat. No. 5,283,845, also incorporated herein by this reference, discloses changing the effective cavity length between two mirrors by varying the axial length of a piezo actuator. This design requires placing two mirrors at the inner ends of two elongated members mounted on glass end plates. An annular or ring piezo actuator extends between the end plates, and is connected via a glass washer at one end and an annular aluminum split ring spacer and a glass washer at the other end. This design is thus unduly complicated. Moreover, because there are only two actuators to control the gap between the two mirrors, the device does not offer fine tuning adjustment to average out the errors caused by non-uniformity of the gap.
It is therefore an object of this invention to provide a tunable optical filter in which the actuating structure which adjusts the spacing between the mirrors does not occlude the optical path through the mirrors.
It is a further object of this invention to provide a tunable optical filter that is less expensive and less complicated than prior art designs.
It is a further object of this invention to provide a tunable optical filter in which the actuator embodiment is not in the optical path between the two mirrors.
It is a further object of this invention to provide a tunable optical filter that avoids the potential for the mirrors to deform or curl.
It is a further object of this invention to provide a tunable optical filter which can be fine-tuned.
It is a further object of this invention to provide a tunable optical filter in which the effects of errors caused by defects in some deformable membrane actuators or arising from other causes can be compensated for by the use of a plurality (array) of individual deformable membrane actuators (actuator cells).
It is a further object of this invention to provide a tunable optical filter in which the flatness of the optical portion is more easily maintained than in the prior art.
This invention results from the realization that an improved tunable optical filter which eliminates the numerous problems associated with prior art tunable optical filters, including light ray absorption, mirror deformation, lack of error compensation, and the inability to fine tune the optical filter, is achieved with a multi-cell deformable membrane actuator located between two substrates, each substrate including optical portions that are moved within closer proximity to each other as the deformable membranes deform (flex) in response to voltage applied to the deformable membrane. Thus, the deformable membrane actuator is separate from, and not a part of any optical portion and the deformable membrane can be located outside of the optical volume while it varies the spacing between the two optical portions.
The present invention provides in one embodiment a tunable optical filter comprising a first optical portion, a second optical portion located a distance from the first optical portion and at least one deformable membrane actuator that is configured to alter the distance between the first optical portion and the second optical portion, the deformable membrane actuator located outside of an optical volume that is defined by the location of the first optical portion and the location of the second optical portion. Optionally, a first primary substrate includes the first optical portion and a second primary substrate includes the second optical portion.
The deformable membrane actuator includes a deformable membrane having electrically conductive properties, a (first) membrane support structure, and an electrode layer forming at least one deformable membrane actuator cell. The membrane support structure being attached to and disposed between the deformable membrane and the electrode layer. Optionally, the electrode layer can be disposed onto the first primary substrate. A voltage applied between the deformable membrane and the electrode layer causes the deformable membrane to deflect towards the electrode layer.
Optionally, the deformable membrane actuator comprises a (second) load support structure which supports a load. In some embodiments, the load is the second primary substrate and the load support structure is attached to the second primary substrate. In these embodiments, the deformable membrane is attached to and disposed between the membrane support structure and the load support structure.
In the preferred embodiment, the deformable membrane actuator comprises a plurality of deformable membrane actuator cells. A deformable membrane actuator cell includes at least a portion of the deformable membrane having electrically conductive properties, at least a portion of the (first) membrane support structure and at least a portion of the electrode layer, the portion of the (first) membrane support structure being disposed between the portion of the deformable membrane and the portion of the electrode layer and forming a deformable membrane actuator well, and where a voltage applied between the portion of the electrode layer and the portion of the deformable membrane causes the portion of the deformable membrane to deflect into the deformable membrane actuator well and towards the portion of the electrode layer.
Optionally, the deformable membrane actuator cell comprises at least a portion of a (second) load support structure which supports a load. In some embodiments, the load is the second primary substrate and the portion of the load support structure is attached to the second primary substrate. In these embodiments, the deformable membrane is attached to and disposed between the portion of the membrane support structure and the portion of the (second) load support structure.
In some embodiments, the membrane support structure includes a plurality of spaced members upstanding from the first substrate and the (second) load support structure includes at least one load support structure member which attaches to the deformable membrane at a location between the spaced members of the membrane support structure.
In some embodiments, the (first) membrane support structures and (second) load support structures each include one or more members that are each disposed at locations along the deformable membrane so that the deformation of the deformable membrane alters the distance between the first primary substrate and the second primary substrate.
In some embodiments, members of the membrane support structure engage the deformable membrane at a first set of locations, members of the load support structure engage the deformable membrane at a second set of locations. Each location of the second set of locations is located in between locations of the first set of locations.
In some embodiments where the support structure is an array of bars, the depth of the membrane support structure may be substantially larger than the height or width of the membrane support structure and may also be substantially larger than the distance of separation between the members of the membrane support structure of the single deformable membrane actuator.
In other embodiments the depth of the membrane support structure may be comparable to the distance of separation between the members of the membrane support structure of a single deformable membrane actuator.
In some embodiments, the load support structure may be an array of individual posts or bars that may in cross-section be square, rectangular, hexagonal, circular, oval, elliptical or other shapes. In these embodiments the depth of the load support structure may be substantially larger than the height or width of the load support structure.
In some embodiments, the tunable optical filter includes least two deformable membrane actuators, each deformable membrane actuator located adjacent to and on opposite sides of the first and second optical portions.
In some embodiments the first optical portion is one or more coatings disposed on the first primary substrate and/or the second optical portion is one or more coatings disposed on the second primary substrate.
In some embodiments, the first optical portion is a discrete optical element located on or within the first primary substrate.
In some embodiments, the second optical portion is one or more coatings disposed on the second primary substrate. In some embodiments, the second optical portion is a discrete optical element located on or within the second primary substrate.
In some embodiments, the first optical portion is coating deposited on a secondary transparent substrate that is embedded in the first primary substrate and/or the second optical portion is coating deposited on a secondary transparent substrate that is embedded in the second primary substrate.
In some embodiments, the first optical portion is a thin-film mirror that bridges a hole or aperture located within the first primary substrate and/or the second optical portion is a thin-film mirror that bridges a hole or aperture located within the second primary substrate.
In some embodiments the first optical portion includes an opposing optical element, such as antireflection coating, disposed on an opposing surface of the first substrate. In some embodiments, the second optical portion includes an opposing optical element, such as antireflection coating, disposed on an opposing surface of the second substrate.
In some embodiments, the first primary substrate is opaque and has a transparent secondary substrate embedded within it and/or the second primary substrate is opaque and has a transparent secondary substrate embedded within it.
In some embodiments, the first primary substrate is a transparent optical flat plate and/or the primary second substrate is a transparent optical flat plate.
In some embodiments the tunable optical filter includes a housing about the first and second substrates, the housing includes a first optical window that is optically aligned with the first optical portion and a second optical window that is optically aligned with a second optical portion.
In some embodiments the electrode layer includes a plurality of independent electrodes and where each of the plurality of electrodes is controlled by a separate voltage or current source. In some embodiments the deformable membrane is segmented into a plurality of contiguous portions where each contiguous portion spans at least one deformable membrane actuator cell. In some embodiments, a group of one or more of the plurality of contiguous portions of the deformable membrane is electronically coupled to one or more of the plurality of independent electrodes.
In some embodiments, the group of one or more of the plurality of contiguous portions of the deformable membrane is electronically coupled exclusively to the one or more of the plurality of electrodes. In some embodiments, the one or more of the plurality of electrodes is controlled by a separate voltage or current source.
In some embodiments the deformable membrane includes a separate electrically conductive layer (membrane electrode). In some embodiments, an electrically conductive layer is deposited onto the portion of the deformable membrane. In other embodiments, the deformable membrane itself is an electrically conductive layer (membrane electrode).
In some embodiments, the membrane support structure comprises at least one wall (ring) disposed above the first primary substrate that forms a membrane actuator well. Optionally, the at least one wall (ring) disposed above the first primary substrate forms a square shaped membrane actuator well. Optionally, the at least one wall (ring) disposed above the first primary substrate forms a hexagonally shaped membrane actuator well.
In some embodiments, the membrane support structure comprises at least one cavity bored into the first primary substrate that forms a membrane actuator well. Optionally, the at least one cavity is arranged into a square pattern. Optionally, the at least one cavity is arranged into a hexagonal pattern. Other patterns can be used.
In some embodiments, the membrane support structure comprises at least one support member disposed above a first substrate that forms at least one membrane actuator well. Optionally, the at least one support member forms a square shaped membrane actuator well. Optionally, the at least one support member forms a hexagonal shaped membrane actuator well. Other shapes can be formed.
The present invention provides in another embodiment, a deformable membrane actuator comprising a deformable membrane having electrically conductive properties, a membrane support structure and an electrode layer, the membrane support structure being disposed between the deformable membrane and the electrode layer and forming at least one well and where a voltage applied between the electrode layer and the deformable membrane causes the deformable membrane to deflect into the well and towards the electrode layer.
Optionally, the deformable membrane actuator includes a load support structure, that is disposed above the deformable membrane and that supports a load located a distance from the electrode layer and where deflection of the deformable membrane alters the distance between the load and the electrode layer.
Optionally, the deformable membrane actuator includes a large plurality of deformable membrane actuator wells to control the movement and position of a load that is substantially heavier than is possible with conventional electrostatic (MEMS) actuator technologies.
The present invention also provides in another embodiment, an array of tunable optical filters having a first substrate including a first plurality of first optical portions, a first plurality of second substrates where each second substrate includes a second optical portion. Each second optical portion is disposed proximate to each first optical portion to form an optical volume through which light travels and includes a second plurality of deformable membrane actuators that are each disposed between the first substrate and each second substrate and disposed outside of each optical volume formed between each said first optical portion and each said second optical portion that is proximate to the first optical portion. Each of the plurality of deformable membrane actuators are configured to alter the distance between each said first optical portion and each said second optical portion.
In some embodiments, the membrane support structure may be an array of individual posts or bars that can have varying cross-sectional shapes. For example, cross-sections may be square, rectangular, hexagonal, circular, oval, elliptical, triangular, triangular with concave sides or other shapes.
In other embodiments, the membrane support structure may be an array of one or more walls that may be square, rectangular, hexagonal, circular, semi-circular, oval, elliptical or other shapes. These walls may be part of individual single-cell actuators, or the walls may be connected side-by-side to form a continuous or discontinuous support web for the membrane or membranes of a multi-cell deformable membrane actuator.
In other embodiments, the membrane support structure may be a substrate with cavities (wells) cut or etched into it. These cavities can have varying shapes. For example, these cavities may be square, rectangular, hexagonal, circular, oval, elliptical or other shapes.
In some embodiments, an electrode layer (substrate electrode) is disposed on the first substrate so as to at least cover the region of the first substrate that lies between members of the membrane support structure. In other embodiments, the first substrate is itself, an electrode.
In some embodiments there is a separate contiguous deformable membrane for each deformable membrane actuator cell. In other embodiments the deformable membrane is continuous (un-segmented) across a plurality of deformable membrane actuator cells. In other embodiments the deformable membrane is segmented into multiple contiguous portions, each contiguous portion is disposed across the plurality of deformable membrane actuator cells. In some embodiments the deformable membrane is segmented into multiple contiguous portions across an elongated deformable membrane actuator well.